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Malformed Request to RPC Endpoint Mapper can Cause RPC Service to Fail (Q305399)

The information in this article applies to:

  • Microsoft Windows NT Server versions 4.0 , 4.0 SP1 , 4.0 SP2 , 4.0 SP3 , 4.0 SP4 , 4.0 SP5 , 4.0 SP6a
  • Microsoft Windows NT Server, Enterprise Edition versions 4.0 , 4.0 SP4 , 4.0 SP5 , 4.0 SP6a
  • Microsoft Windows NT Workstation versions 4.0 , 4.0 SP1 , 4.0 SP2 , 4.0 SP3 , 4.0 SP4 , 4.0 SP5 , 4.0 SP6a
  • Microsoft Windows NT Server versions 4.0 , 4.0 SP4 , 4.0 SP5 , 4.0 SP6 , , Terminal Server Edition


The RPC (Remote Procedure Call) endpoint mapper allows RPC clients to determine the port number currently assigned to a particular RPC service. Because the endpoint mapper runs within the RPC service itself, exploiting this vulnerability would cause the RPC service itself to fail, with the attendant loss of any RPC-based services the server offers, as well as potential loss of some COM functions. Normal service could be restored by rebooting the system.

Standard security recommendations call for port 135 - the port on which the RPC endmapper operates - to be blocked at the firewall. If this were done, the chief threat posed by this vulnerability would be from internal attacks.


This vulnerability results because the Windows NT 4.0 Remote Procedure Call (RPC) service will fail if the endpoint mapper is sent a request that contains a particular type of malformed data.


A supported fix is now available from Microsoft, but it is only intended to correct the problem described in this article and should be applied only to systems that are determined to be at risk of attack. Please evaluate your computer's physical accessibility, network and Internet connectivity, and other factors to determine the degree of risk to your computer. Please see the associated Microsoft Security Bulletin to help make this determination. This fix may receive additional testing at a later time, to further ensure product quality. If your computer is sufficiently at risk, Microsoft recommends that you apply this fix now.

To resolve this problem immediately, download the fix as instructed below or contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:
NOTE : In special cases, charges that are normally incurred for support calls may be canceled, if a Microsoft Support Professional determines that a specific update will resolve your problem. Normal support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

The following file is available for download from the Microsoft Download Center:
] Download Q305399i.exe now

NOTE : A patch for Windows NT Server 4.0, Terminal Server Edition is forthcoming.
Release Date: September 10, 2001

For additional information about how to download Microsoft Support files, click the article number below to view the article in the Microsoft Knowledge Base:
Q119591 How to Obtain Microsoft Support Files from Online Services
Microsoft used the most current virus detection software available on the date of posting to scan this file for viruses. Once posted, the file is housed on secure servers that prevent any unauthorized changes to the file.

The English version of this fix should have the following file attributes or later:
   Date         Time   Version        Size     File name
   09-Aug-2001  10:56  4.0.1381.7097  701,200  Ole32.dll
   09-Aug-2001  10:56  4.0.1381.7097  343,312  Rpcrt4.dll
   16-Aug-2001  12:01  4.0.1381.7097  107,280  Rpcss.exe
NOTE : Due to file dependencies, this hotfix requires Microsoft Windows NT 4.0 Service Pack 6a (SP6a).


Microsoft has confirmed that this problem could result in some degree of security vulnerability in Windows NT 4.0.


For more information on this vulnerability, see the following Microsoft Web site:

Published Sep 11 2001 6:12AM Issue Type kbbug
Last Modifed Sep 11 2001 8:36AM Additional Query Words security_patch DoS denial of service
Keywords kbSecurity kbWinNT400PreSP7Fix


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